Effect of large-heat-capacity substances doping of superconducting coils on their dynamic stability

Abstract

The dependence of the quench currents on the ramp rate was studied for four small NbTi coils. Two pairs of superconducting coils were tested. In one pair the SC 0.85-mm-dia wire with 2970 filaments was used, in the other two coils the SC wire contained 8910 filaments of smaller size. Two coils (with different number of filaments) contained 4.9 vol % of Large-Heat-Capacity Substance (LHCS) in the form of tiny powder evenly distributed over the winding volume, therefore their heat capacity at 4.2 K was an order of magnitude larger than that for coils without LHCS. The LHCS was introduced into the winding in a mixture with epoxy resin (“wet” winding). When the self-magnetic field varied with a rate of ≥5 T/s, premature quenches were observed in the central turns of the undoped coil made of a wire with 2970 filaments. These transitions are likely to be caused by magnetic flux jumps. In the LHCS-doped coil made of the same wire, the quenches took place at currents two to three times higher, since the sample was heated up to a critical temperature because of electrical losses (as confirmed by calculations). Thus, the improved stability of the LHCS-containing coils not only against long-term (0.1–1.0 s) disturbances but also against much shorter (10–100 μs) jumps of the magnetic flux is demonstrated.